1. Field of the Invention
The present invention relates to purification and filtration of polymeric materials and, more particularly, to an improved method and apparatus for diverting flow of polymeric materials between at least two filter housing assemblies utilizing a single valve in a continuous polymer extrusion process.
2. Description of the Related Art
In processes involving extrusion of molten thermoplastic polymers such as polyethylene, Nylon, polyester, polystyrene, etc., it is necessary to filter foreign matter, e.g., contaminants and impurities, from the molten polymer. Two common examples of articles manufactured by molten polymer are synthetic textile fibers and thin plastic films of the type used in packaging or for tapes (e.g., computer tape, sound recording tape, etc.). In the production of synthetic textile fibers, which may have a final diameter of as little as ten microns, a particle of foreign matter of five or more microns diameter is quite likely to cause breakage of the fiber during manufacturing. It is desirable, therefore, to filter out any foreign material above a certain size.
In the manufacture of polymers for use in fiber production, a final step is often a pelletizing operation where a filter is used to remove any impurities from the final pellets. One form of impurity is a so-called “gel”, a region in the molten polymer which has a much higher than average viscosity due to excessive polymer molecular weight or cross-linking of polymer molecules. For some articles, such as very thin film and textile fibers, gels in the molten polymer degrade the product quality and are desirably removed by filtration, either at the time the polymer is manufactured, or in-line in an extrusion process upstream of formation of the final product. It is further desirable to be able to replace dirty filter media with clean filter media without interrupting an extrusion process and without introducing air to the molten polymer fluid stream, since air introduced to the stream causes bubbles in the extruded article, rendering the article defective. A polymer filter having the capability of changing media without interrupting the extrusion operation is generally called a “continuous polymer filter” or “continuous screen changer”.
Many polymer filtration systems are known in the art for removing impurities from molten polymer. One common type of polymer filter is a screen changer system. An example of a screen changer system is described in U.S. Pat. No. 3,007,199 to Curtis, the disclosure of which is incorporated herein by reference in its entirety. Screen changers typically have a relatively small area of screen for a given flow rate of molten polymer. A filter area of one square inch of screen for flow rates of thirty to seventy pounds per hour of polymer is typical of screen changers. For instance, an extruder of 4.5 inches screw diameter at full production can melt 600 to 1100 pounds per hour of polymer and is commonly fitted with a screen changer using screens of 4.5 inches diameter or 15.9 square inches of filter area. This yields a flow rate of approximately thirty-eight to sixty-nine pounds per hour per square inch of filter area. It would be very unusual but possible to use a screen changer with screens bigger than eight inches diameter on a 4.5 inch extruder. This would yield fifty square inches of area or twelve to twenty-two pounds per hour of polymer flow per square inch of filter area. In any case, screen changers of this type are suitable to remove dust, dirt, metal particles and pigment agglomerates down to a micron size of about forty, more often one hundred microns.
Gel removal, on the other hand, requires filtration by media having a micron rating of twenty or finer, and typically a greater “depth” or thickness of media is used than when merely removing dirt. Gels are amoeba-like in that they can change shape to pass through normal filter screens and then resume a more compact shape downstream of the screen. The combination of finer media and a greater thickness of media tends to cause a very high pressure drop through the filter unless a large area of filter media is used. For this reason, filters for gel removal normally have one square inch of filter area for each 0.20 to 0.70 pounds per hour of polymer flow. A filter used with a 3.5 inch diameter screw extruder having a melt rate of 350 pounds/hour would have a filter media area of about 1300 square inches, or nine square feet. These large area filters are not only useful for molten polymer, but also for solutions of polymer (so-called dopes). Polymer solutions (as used to make spandex or acrylic fibers) are lower in viscosity than polymer melts, so somewhat less filter area is needed to remove the gels that are common in these dopes. Also, filters for polymer solutions often operate at ambient temperature, making it unnecessary to heat the filter apparatus.
While many varying types of screen changers and other small area polymer filters exist, most large-area gel filters have a similar construction and utilize a candle filter system. A candle filter system typically has two or more filter housings and uses valves to direct polymer to and from the filter housings. Each housing typically contains multiple candle-type filter elements. The candle filter element is typically a perforated tube covered by pleated screen wire in two or more layers. The candle filter system is normally used for high polymer flow rate and/or very fine filtration systems. The size and number of candle filters are selected based upon the desired flow rate of polymer fluid to be processed. An example of a candle filter system is described in U.S. Pat. No. 3,833,121 to Singleton et al., the disclosure of which is incorporated herein by reference in its entirety. One popular candle size is 1.38″ O.D. by 16″ long and has 1.2 to 1.4 square feet of area, or about 9 square feet (1300 square inches) for seven candles. Such a filter can be used with a polymer flow rate of two hundred to one thousand pounds per hour, or 0.20 to 0.77 pounds per hour per square inch of area. This corresponds to the output of an extruder with a screw diameter of 2.5 to 4.5 inches.
U.S. Pat. No. 5,462,653 to Hills, the disclosure of which is incorporated herein by reference in its entirety, discloses another candle filtration system utilizing a single housing to filter polymer fluid. Briefly, the Hills system includes a generally cylindrically shaped housing with six candle-type filter assemblies arranged in pairs in a ring about a central valve and distribution system. A rotatable control plate controls the valve and distribution system and can be set in various positions to allow polymer flow through all of the filter assemblies or to prevent flow through individual pairs of filter assemblies while the other assemblies remain on-stream, in order to permit removal or replacement of clogged or dirty filters. While the system of Hills is useful in diverting polymer flow through various candle filter assemblies within a particular housing, the system does not disclose any mechanism for maintaining continuous filtration of polymer fluid in the event the entire filter housing needs to go off-stream for cleaning.
Many candle-type polymer filter systems maintain one of two filter housings operable or on-stream while the other is cleaned, installed and heated to be ready to accept the polymer when the filter medium in the on-stream housing becomes too dirty for continued operation. To switch housings, at least two valves are operated simultaneously (or nearly simultaneously) and polymer is introduced to the clean housing while flow continues through the dirty housing. An example of such a candle-type filter system is the Fluid Dynamics CPF system, which is manufactured by USF Filtration & Separation, Inc. This system has two filter housings and uses two sliding spool valves to direct the polymer flow to and from the filter housings. During normal operation, one of the two filter housings is on-stream (i.e., molten polymer is flowing therethrough) while the other filter housing is cleaned, installed and heated to be ready to accept the polymer. When the on-stream filter becomes too dirty for continued operation, spool valves of the system are set in motion in the following sequence: (1) the inlet valve of the clean filter housing is slightly opened while the outlet valve of the clean filter housing remains closed to allow the polymer fluid to enter and fill the clean housing; (2) the trapped air in the clean housing is purged through a bleed port until all air is vented from the clean housing; (3) after the clean housing is completely filled with molten polymer, the bleed port is closed and then the outlet valve of the clean housing is fully opened; and (4) the inlet valve of the clean housing is fully opened, after which the inlet and outlet valves of the dirty housing are completely closed. This completes the switching of the polymer fluid from the dirty housing to the clean housing, and the filter of the dirty housing can then be removed for cleaning or replacement. While the clean housing is being filled, the filter element in the dirty housing continues to provide uninterrupted process filtration.
One typical problem associated with typical candle filter systems is the lack of uniformity in the heating of candle housings. Another problem with candle filter systems, such as the CPF system described above, is the occurrence of excessive residence time for the polymer in the filter housings if the unit is operated well below its maximum flow capacity. Normally, the size of a polymer filtration system is chosen to provide sufficient filtration for the polymer process system at its maximum flow rate. The area of the filter media elements utilized to filter the polymer fluid will determine the polymer flow rate capacity suitable for the filter system. Under certain operating conditions or for certain processes, the process system maybe required to run at a reduced capacity, for example, in a process system having multiple functions or in systems producing plural-component polymer products. One problem resulting from running the process system at a reduced capacity or variable capacity is that the molten polymer remains within the filtration system for a relatively long period of time (i.e., the polymer has a high polymer “residence time”).
Excessive residence time and non-uniform residence time can cause thermal degradation of the polymer, particularly when thermally sensitive polymers are used. For example, one type of synthetic fiber producing apparatus produces fibers with two polymer components, so-called bicomponent fibers. Such a machine would have an extruder and a filter for each polymer. If the machine were making a common type of bicomponent fiber with a core of one polymer and a sheath of another, it is desirable to be able to vary the percentage of sheath polymer for 10% to 60%, while the core would vary from 40 to 90%. In the case of the sheath, this is a 6 to 1 variation in polymer flow, causing excessive filter residence time when a thin sheath is being produced. To solve this problem, it is necessary to replace the filter housings with ones having less volume and fewer or shorter candle elements. Shorter elements require a shorter housing to reduce volume, but a shorter housing is not practical on all existing filter assemblies because the two valves and the polymer piping connected to the valves is a fixed distance apart and will only accommodate a housing of one specific length.
U.S. Pat. No. 6,221,266 to Wilkie et al., the disclosure of which is incorporated herein by reference in its entirety, discloses a continuous polymer filtration system that solves the problem of excessive residence time that occurs with varying polymer flow rates. The system in Wilkie et al. includes at least three independently controllable and removable filter housings that extend from a common inlet passage to a common outlet passage. Each filter housing includes a valve at its inlet and another valve at its outlet that may be opened or closed independently of the valves for the other housings. By manipulating various valves, the Wilkie et al. system is capable of diverting polymer fluid flow through different filter housings of the filtration system to effectively reduce excessive residence time when polymer flow rates change.
The Wilkie et al. system is similar to the CPF system and many other candle filter systems employing multiple filter housings in that multiple valves are utilized to control fluid flow within the system. The utilization of multiple valves to divert polymer fluid streams from one filter housing to another can be complicated. For example, operator error in switching polymer fluid flow between housings can cause polymer flow to be completely shut off to all housings, which stops flow from the extruder that delivers polymer to the filter system and causes a safety rupture disc typically installed at the exit of an extruder to rupture. In such situations, the entire process must be halted until the rupture disc is replaced.
Thus, it is desirable to provide a polymer filtration system capable of continuously filtering polymer streams at varying flow rates to filter housings while maintaining a desired polymer residence time within the filter housings. It is further desirable to minimize the risk of operator error and damage to the system when diverting polymer fluid streams between two or more filter housings.